OpenBCI Ganglion board instrumental amplifiers

Hello OpenBCI,

I want to know why in ganglion board, there's additional instrumental amplifier (AD8237) for each input channel not just PGA of the ADC like in Cyton board?

thanks in advannce.


  • Hi hamza,

    I can't speak for openbci designers & others knowledgeable about all the choices, but here's one pointer ....
    The Ganglion ADC-etc-chip MCP3912, desirable for Ganglion partly because of lower-cost than Cyton's, has fairly low input impedance, which also varies with PGA gain.  See the chip's spec page 4, Table 1-1 for Differential Input Impedance.

    Other advantages offered by the instrumentation amp rather than straight-into-MCP3912 are the opportunity for gain (chosen 51 at mid-band) and the opportunity to include high-pass filtering, and it eases inclusion of low-pass filtering before the 3912 without affecting board-input-impedance.  There may be other advantages I'm not seeing.

    The Cyton ADC was designed specifically for direct-connect bioinstrumentation so it has much better features & performance for that.  The Ganglion ADC seems to have initially been designed primarily for AC power analysis applications.

        -- Bruce P.

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